316 EDWARD PHELPS ALLIS, JR. 
rior proper. This fenestra is a perforation of the roof of the 
myodomic cavity, and hence is not the homologue of the so- 
called fenestra basicranialis posterior of embryos of fishes, which 
is a perforation of the floor of that cavity. This latter fenestra 
of embryos of fishes is the homologue of the fenestra hypophyseos 
of birds and mammals, the so-called anterior prolongations of 
the parachordals of fishes being the homologues of the polar 
cartilages of birds and mammals. 
In certain of the Selachii there is an acustico-trigemino-faci- 
alis recess, and there may be certain canals in the cranial wall 
traversed by the vena jugularis and the external carotid artery. 
In Amia the trigemino-facialis portion of this recess has fused 
with the canals for the vena jugularis and the external carotid 
artery to form a trigemino-facialis chamber; this chamber has 
become continuous with the myodome, and the large chamber 
so formed has been prolonged anteriorly by a space between the 
pedicel of the alisphenoid and the primitive side wall of the 
neurocranium. The foramina for the pituitary vein and the 
oculomotor and trochlear nerves open into this anterior pro- 
longation of the chamber, and through its orbital opening into 
the orbit. The vena jugularis traverses this opening to enter 
and traverse the trigemino-facialis chamber; the musculus rectus 
externus traverses it to enter the myodome, and the nervus pro- 
fundus traverses it to join the ganglion, or root of the nervus 
trigeminus. The nervus trigeminus and the external carotid 
artery issue from the trigemino-facialis chamber posterior to 
the pedicel of the alisphenoid and run forward lateral to it. 
In the non-siluroid Teleostei the trigemino-facialis chamber 
is not continuous with the myodome, and it has been separated 
by a wall of bone into ganglionaris and jugularis parts which 
correspond, respectively, to the trigemino-facialis recess and the 
jugular and external carotid canals of the Selachii. The pedicel 
of the alisphenoid is incomplete or wholly wanting, but it may 
be replaced by an anterior prolongation of the ascending process 
of the parasphenoid. In the latter case the nerves, arteries, 
veins, and muscles all have the same relations to this process 
that they have to the pedicel of the alisphenoid of Amia. ‘The 
